We propose to examine changes with aging in both male and female rats in the expression of two genes involved in intestinal calcium absorption and in vitamin D regulated renal handling of calcium; calbindin (the vitamin D- dependent calcium binding protein) and the vitamin D receptor (VDR). Our hypothesis is that aging results in an alteration in the vitamin D endocrine system and in 1,25 dihydroxyvitamin D (1,25(OH)3D3) regulated gene expression which results in a deterioration in the normal adaptive response to inadequate calcium intake or to calcium need. Effective calcium deficiency as a result of defective l,25(OH)2D3 regulated gene expression becomes a potentially more and more important problem with advancing age, resulting in age related bone loss aggravated by calcium deficiency. The first specific aim is to determine the effect of age and sex on intestinal vitamin D3 receptor and calbindin gene expression and to determine how this relates to bone mineralization. Protein and mRNA levels (by Northern and slot blot analysis) for receptor calbindin will be measured in 3, 6, 12 and 24 month old male and female rats. Serum calcium and bone mineralization will also be evaluated. The second specific aim is to determine the effect of age and sex on intestinal and renal responsiveness to l,25(OH)2D3 or vitamin D3 and on adaptation to changes in dietary calcium. Three, 6, 12 and 24 month old male and female rats will be fed a strontium diet (0.8%) for 6 days to induce a functional l,25(OH)2D3 deficiency. Animals will be sacrificed at various times after a single intraperitoneal dose of l,25(OH)2D3. Intestinal calbindin and 1,25(OH)2D3 receptor protein and mRNA will be analyzed. In order to determine the effect of age and sex on adaptation to dietary calcium, male and female rats at 3, 6, 12 and 24 months will also be fed low and high calcium diets for 10-30 days. Changes in intestinal and renal calbindin protein and mRNA and in bone mineralization will be evaluated. No manuscripts have been published concerning changes in female rats in intestinal calbindin and 1,25(OH)2D3 receptor gene expression during aging. It is indeed probable that there is some degree of age related resistance of the intestinal mucosa and kidney to the action of 1,25(OH)2D3 during the aging process. It is now timely to determine changes in vitamin D regulation with aging. A greater understanding of vitamin D-dependent adaptation in older female as well as male animals will enable us to obtain information on how various stages of maturation and aging can affect bone density and vitamin D-dependent gene expression. These studies could lead to future investigations concerning genetic manipulation of calcium responsiveness during aging.